Most scenic train rides covered by Berner Oberland pass

You could take a train to Luzern, which is a very scenic ride. Spend the afternoon in Luzern and come back. If you really wanted to make a day of it, you could start with a boat ride from Interlaken Ost station to Brienz, then catch the train to Luzern.

We made great use of our BO Reg Passes last fall when we had a number of less-then-ideal hiking days. I wrote it up on my blog but here is the extract I think most pertinent to your question. We called it “Mountain Joy Riding.”

Taking advantage of our Berner Oberland Regional Passes, we did a grand tour of the Jungfrau Region. Here are the stages:

• Wengernalpbahn (train) to Kleine Scheidegg at 2061 meters/6762 feet of altitude
• Change trains at the top to the Wengernalpbahn down to Grindelwald
• Lunch and browsing in Grindelwald
• Change trains in Grindelwald to the BOB and ride to Grindelwald Terminal base station for two fabulous lifts, the new Eiger Express and the Männlichen Gondolbahn
• We took the Gondelbahn to Männlichen an amazingly long cableway
• Männlichen to Wengen via another cableway

Our excursion actually started and ended in Lauterbrunnen where we were staying. This trip would cost CHF 134.00, but with the pass the cost was zero.

Adding tomLaurel’s suggestion—in Grindelwald, you could walk through town to the First gondola. Ride up to the first mid-station at Bort, and have lunch on the sun terrace at the restaurant there ( instead of a picnic).

https://www.huettenzauber.ch/en/hotels-accommodation/alpinhotel-bort
Or ride to the top and have lunch at the restaurant there:

https://berggasthausfirst.ch/en/

After lunch, you could continue upward on the gondola to the top and do the First Cliffwalk ( no charge).

https://www.jungfrau.ch/en-gb/grindelwaldfirst/first-cliff-walk-by-tissot/

https://fullsuitcase.com/first-cliff-walk-grindelwald/

All of this ( except the lunch) is covered with the Berner Oberland Pass.

Other activities from the First gondola, such as the zip line or Trotti-bikes, are available for an extra charge.

https://www.jungfrau.ch/en-gb/grindelwaldfirst/

For scenic train rides covered by the BO Pass I would put them in this order:

1. Via Meiringen to Luzern (as Sam suggested) - this is especially beautiful because of the combo of lakes and mountains
2. Heading in the other direction on the Golden Pass route via Spiez to Gstaad
3. Via Speiz to Kandersteg - not one of the official scenic train routes, but still very beautiful and you can spend a bit of time in Kandersteg if you want

I would not say that these are essentially better than the great suggestions given regarding exploring the mountain lifts, etc. Just more options if you want to go further afield on the train (as you mentioned).

If you choose Kandersteg, walk through town to the gondola that goes up to the lake. There is a Rodelbahn ( summer live) at the top of the gondola. The walk to Oeschinensee itself —a lake known as the “jewel of the Alps”—-is about 20 minutes. You can bring food for a picnic, or have lunch on the deck of the mountain inn on the lakeshore, or pick up pre-ordered supplies and food for a grill party (see “barbeque” on the page below).

https://www.oeschinensee.ch/en/sommer/

You can also rent rowboats and get out on the lake.